I was waiting for him to get called up, didn't make it to the free agent board in time when I heard he got called up. I was offered immediately a trade from the guy that did pick him up. My Martin Prado for Mike Trout. Should I do this?

Thats a good deal for you but, if I owned Trout, I wouldn't accept. We basically know what Prado is at this point and that is basically just a mediocre option in LF and 3B (he isn't eligible at 2B in most leagues). He is a small price to pay for Trout in a keeper league, as Trout is worth more. Heck, even in a redraft league I would value Trout more because I think he has a very legit shot at outproducing Prado for the remainder of the season.